1. Field of the Invention
The present invention relates to a liquid crystal display device whose capacity of a frame memory required for driving a liquid crystal panel in an overshooting manner can be reduced.
The present application claims priorities of Japanese Patent Application Nos.2001-192076 filed on Jun. 25, 2001 and 2002-146165 filed on May 21, 2002, which are hereby incorporated by reference.
2. Description of the Related Art
Transmittance of light of a liquid crystal substance making up a liquid crystal cell is changed by a change in an alignment of a molecule occurring when an electric field is applied. An image display can be achieved by using a liquid crystal panel in which many tiny liquid crystal cells made up of liquid crystal substances are arranged on a transparent substrate and a signal voltage can be individually applied to each of the liquid crystal cells and by using a light source mounted on a rear of the liquid crystal panel and by controlling transmittance of light applied from the light source for every liquid crystal cell.
However, in the liquid crystal display device, the change in the molecular alignment occurring at a time when an electric field is applied to the liquid crystal substance entails time delays and therefore an accumulating effect in its light-emitting responsivity is produced. As a result, in the case of displaying moving pictures, there is a problem in that delays in the movement of the image cause a person to be hard to see pictures.
An effort to improve display performance of a moving picture is being made by employing an overshooting driving method in which a large signal voltage is applied for a short time while a liquid crystal cell is driven which causes a change in a molecular alignment of a liquid crystal substance to be accelerated.
FIG. 14 is a schematic block diagram showing an example of a configuration of a conventional liquid crystal display device to be driven in an overshooting manner. The conventional liquid crystal display device, as shown in FIG. 14, chiefly includes a controller 101, a frame memory 102, a look-up table 103, and a liquid crystal display (LCD) 104. Moreover, in the liquid crystal display device, in ordinary cases, liquid crystal cells each corresponding to each of three primary colors including a red (R) color, green (G) color, and blue (B) color are provided and RGB data is fed to each of the liquid crystal cells to display a color image and hereinafter operations in the case of a single color are described to simplify the explanation.
An input 1 for an image signal made up of, for example, bits of digital data (gray-scale value) is fed from an outside device to a controller 101 and is sequentially transferred to a frame memory 102 where it is held for one frame period and then is output. The controller 101 feeds an output from the frame memory 102 as an input 2 to the look-up table 103.
On the other hand, the input 1 is directly fed to the look-up table 103. The look-up table 103, according to a gray-scale value of the input 1 and the input 2 generates an output 2 required for an overshooting driving and feeds it to the LCD 104.
In the LCD 104, a pixel electrode is mounted at every point of intersections of a plurality of scanning lines arranged in a horizontal (row) direction and a plurality of data lines in a vertical (column) direction. It has a scanning line driving circuit used to drive the scanning line (not shown) and a data line driving circuit used to drive the data line (not shown). A supply of a scanning signal from the scanning line driving circuit according to synchronizing data fed from the controller 101 causes the scanning line on each row to be sequentially driven and a supply of a data signal by the data line driving circuit to each data line according to synchronizing data fed from the controller 101 and according to a gray-scale value of the output 2 fed from the look-up table 103 causes the data line on each column to be sequentially driven. An image display is achieved by changing transmittance of light according to a voltage of a data signal fed from a corresponding data line occurring when a gate of a TFT (Thin Film Transistor) being connected between each of the pixel electrodes and each of corresponding data lines is turned ON by a scanning signal from the scanning line.
At this point, the look-up table 103, in order to perform an overshooting driving, generates the output 2 being an overshooting gray-scale value, for one frame period following a change of a gray-scale value of the input 1 according to a gray-scale value of the input 1 and input 2. That is, the look-up table 103 is set a value, in advance, so that, when a gray-scale value of an input 1 is equal to a gray-scale of an input 2 the gray-scale value is output as an output 2, however, when a gray-scale value of the input 2 is smaller than a gray-scale of the input 1, an output 2 having a gray-scale value being larger than an gray-scale value of an input 2 is output as an overshooting gray-scale value and, when a gray-scale value of an input 2 is larger than a gray-scale value of an input 1, an output 2 having a gray-scale value being smaller than a gray-scale value of the input 2 is output as an overshooting gray-scale value.
Functions of the overshooting driving are described by referring to FIG. 15 below. When there is no change in a gray-scale value in the input 1 during a frame Fl, since no change occurs yet in the input 2, signal levels of the input 1 and input 2 are same in the look-up table 103 and, therefore, a gray-scale value of the input 1 =input 2 =D1 is output as an output 2 from the look-up table 103. During a subsequent frame F2, when a gray-scale value of the input 1 is changed from Dl to D2, since a gray-scale value Dl of the input 2 remains unchanged, the input 1 to the look-up table 103 >the input 2 to the look-up table 103 and an overshooting value DO being stored in advance is output corresponding to a gray-scale value D1 of the input 1 and a gray-scale value D2 of the input 2 is output as an output 2 from the look-up table 103, which causes the overshooting driving to be performed.
In a subsequent frame F3, since a gray-scale value of the input 2 becomes D2, a gray-scale value of an input 1 =input 2 =D2 is output as an output 2 from the look,-up table 103. A data signal for a corresponding pixel electrode is applied to the LCD 104 according to a gray-scale value fed from the look-up table 103 and, at this point, luminance of the LCD 104 is changed in such a manner that its luminance L1 corresponding to a gray-scale value D1 during the frame F1 undergoes a transient change based on an overshooting gray-scale value D0 during the frame F2 and becomes a luminance L2 corresponding to a gray-scale value D2 during the frame F3.
Now, if it is presumed that, in FIG. 14, there is no look-up table 103 and the LCD 104 is to be directly driven by the input 1, as indicated by a broken line in FIG. 15, a gray-scale value of the output 2 to be fed to the LCD 104 immediately becomes “D2” during the frame F2, however, a rise of the luminance of the LCD 104 from its luminance L1 is gentle due to delays in operations of the liquid crystal cell as indicated by another broken line in FIG. 15 and very long delays develop in an image display.
In contrast, when the overshooting driving is employed, as indicated by a solid line in FIG. 15, the rise of the luminance of the LCD 104 from its luminance L1 corresponding to the gray-scale value D1 becomes sharp and then the luminance corresponding to the gray-scale value D2 becomes “L2”, thus improving delays in the image display.
Moreover, if an input gray-scale value D2 is larger than an input gray-scale value D1, an overshooting gray-scale value D0 becomes larger than an input gray-scale value D2. However, it the input gray-scale value D2 becomes smaller than an input gray-scale value D1, an overshooting gray-scale value D0 becomes smaller than the input gray-scale value D2. The larger becomes a difference between the input gray-scale value D2 and the input gray-scale value D1, the larger becomes an amount of a change in the overshooting gray-scale value D0 relative to the input gray-scale value D2.
Thus, in the liquid crystal display device, by performing overshooting driving using the look-up table to improve delays in the image display, it is made possible to improve visibility in image display of moving pictures.
However, the conventional liquid crystal display shown in FIG. 14 has a problem in that, since data of the input 1 fed from an outside device, as it is, is directly stored in the frame memory 102, it is necessary that memory capacity of the frame memory 102 should be large.